The method of sewage treatment commonly employed in North America consists of the following sequence of steps: Firstly, screening out coarse solids, such as tin cans, rags and paper. This is usually done through mechanical raking over a 24 hour period. Secondly, letting the sewage stand for a period of approximately 4 days to permit sedimentation. Thirdly, aerobically and anaerobically treating the sewage over a period of approximately 4 days by introducing an abundace of oxygen into the sewage to kill anaerobic bacteria and then introducing microorganisms into the sewage which deprive the sewage of oxygen to kill aerobic bacteria. Fourthly, diverting the sewage into secondary holding tanks and letting the sewage to stand for a further period of approximately 4 days to permit sedimentation. Fifthly, passing the liquid effluent through a series of lagoons into which chemicals are added and ultimately into the nearest body of water to reduce its potency. Sixthly, dehydrating the sludge over a 24 hour period, conditioning it with the addition of chemicals and composting with domestic refuse in a land fill site. There is a further step of filtration to which the liquids are subjected in some systems which takes an additional day, however, this step is often omitted to save expense.
The described method of sewage treatment takes approximately 14 days to complete and permits fungi and many disease causing microorganisms to survive. The survival of the fungi and microorganisms presents an environmental hazard as it results in many animal diseases (such as polio) and most plant diseases, being spread through our sewage treatment systems. This results in a waste of water as liquid effluent cannot be used to irrigate plants which are ultimately intended for human consumption.